Valorization of Bambara groundnut shell via intermediate ... Valorization of...5 and Feroz Kabir...

Valorization of Bambara groundnut shell via intermediate pyrolysis: Products distribution and characterizationMOHAMMED, Isah Yakub, ABAKR, Yousif Abdalla, MUSA, Mukhtar, YUSUP, Suzana, SINGH, Ajit and KABIR, Feroz

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MOHAMMED, Isah Yakub, ABAKR, Yousif Abdalla, MUSA, Mukhtar, YUSUP, Suzana, SINGH, Ajit and KABIR, Feroz (2016). Valorization of Bambara groundnut shell via intermediate pyrolysis: Products distribution and characterization. Journal of Cleaner Production, 139, 717-728.

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Valorization of Bambara Groundnut Shell via Intermediate Pyrolysis : Products 1

Distribution and Characterization 2 3

Isah Yakub Mohammeda,f,*

, Yousif Abdalla Abakrb, Mukhtar Musa

c,f, Suzana Yusup

d, Ajit Singh

c 4

and Feroz Kabir Kazie

5

6 aDepartment of Chemical and Environmental Engineering, the University of Nottingham 7

Malaysia Campus, Jalan Broga, Semenyih 43500, Selangor Darul Eshan, Malaysia

8

9 bDepartment of Mechanical, Manufacturing and Material Engineering, the University of 10

Nottingham Malaysia Campus, Jalan Broga, Semenyih 43500, Selangor Darul Eshan, Malaysia

11

12 cSchool of Biosciences, the University of Nottingham, Malaysia Campus, Jalan Broga, 43500 13

Semenyih, Selangor, Malaysia 14

15 dDepartment of Chemical Engineering, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 16

31750, Tronoh, Malaysia 17

18 eDepartment of Engineering and Mathematics, Sheffield Hallam University, City Campus, 19

Howard Street, Sheffield S1 1WB, UK 20

fCrops for the Future (CFF), the University of Nottingham Malaysia Campus, 21

Jalan Broga, Semenyih 43500, Selangor Darul Eshan, Malaysia 22

23

Abstract 24

This study provides first report on thermochemical conversion of residue from one of the 25

underutilized crops, Bambara groundnut. Shells from two Bambara groundnut landraces KARO 26

and EX-SOKOTO were used. Pyrolysis was conducted in a vertical fixed bed reactor at 500, 27

550, 600 and 650 oC; 50

oC/min heating rate and 5 L/min nitrogen flow rate. The report gives 28

experimental results on characteristic of the feedstock, impact of temperature on the pyrolysis 29

product distribution (bio-oil, bio-char and non-condensable gas). It evaluates the chemical and 30

physicochemical properties of bio-oil, characteristics of bio-char and composition of the non-31

condensable gas using standard analytical techniques. KARO shell produced more bio-oil and 32

was maximum at 600 oC (37.21 wt%) compared to EX-SOKOTO with the highest bio-oil yield 33

of 32.79 wt% under the same condition. Two-phase bio-oil (organic and aqueous) was collected 34

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and analyzed. The organic phase from both feedstocks was made up of benzene derivatives 35

which can be used as a precursor for quality biofuel production while the aqueous from KARO 36

consisted sugars and other valuable chemicals compared to the aqueous phase from EX-37

SOKOTO which comprised of acids, ketones, aldehydes and phenols. Characteristics of bio-char 38

and composition of the non-condensable were also determined. The results show that bio-char is 39

rich in carbon and some minerals which can be utilized either as a solid fuel or source of bio-40

fertilizer. The non-condensable gas was made up of methane, hydrogen, carbon monoxide and 41

carbon dioxide, which can be recycled to the reactor as a carrier gas. This study demonstrated 42

recovery of high quality fuel precursor and other valuable materials from Bambara groundnut 43

shell. 44

Keywords: Bambara groundnut shell; Pyrolysis; Bio-oil; Bio-char; Non-condensable gas; 45

Characterization 46

*Corresponding author: Isah Yakub Mohammed ([email protected]) 47

Abbreviations AAK Acid, aldehyde, ketones

AMH Atomic mass of hydrogen

AMO Atomic mass of oxygen

C Carbon

CFF Crops for the future

DTG Derivative thermogravimetric

EDX Energy dispersive x-ray

ES Esters

FTIR Fourier transform infrared

GC Gas chromatograph

GC-MS Gas chromatograph-mass spectrometer

H Hydrogen

HC Hydrocarbon

HHV Higher heating value

KBr Potassium bromide

LHV Lower heating value

LHw Latent heat of vaporization of water

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N Nitrogen

O Oxygen

PH Phenolics

RKO Karo Bambara shell

S Sulfur

SGR Sugars

SKT EX-SOKOTO Bambara shell

TCD Thermal conductivity detector

TG Thermogravimetric

TGA Thermogravimetric analyzer

VAC Value added chemicals

XRD X-ray diffraction 48

49

1. Introduction 50

Bambara groundnut (Vigna subterranea (L.) Verdc), is an underutilized pulse legume cultivated 51

primarily for its seeds, which are consumed in various forms of dishes, rich in protein (16-25 %), 52

carbohydrate (42-60 %), lipid (5-6 %) (Brough and Azam-Ali, 1992), fibre (4.8 %) and ash (3.4 53

%) (Brink et al., 2006). The fresh pods containing the seeds are boiled and eaten as snacks 54

whereas the haulm, is used as livestock feed in most of the drier parts of Africa (Heller et al., 55

1997). Legume such as Bambara groundnut with the ability to meet greater % of their N 56

requirement as a result of atmospheric N fixation have been identified as potential alternative 57

crops for mitigating climate change (Jensen et al., 2012) and offer an important alternative to the 58

use of inorganic N fertilizers that are costly and unavailable to resource-poor farmers. The crop 59

is predominantly grown by subsistence farmers over much of semi-arid Africa and is reported to 60

spread into new production areas such as Indonesia, Malaysia and Thailand in the Asian 61

continent and is increasingly becoming popular due to its ability to thrive under conditions which 62

are too poor for other tropical legumes such as cowpea and groundnut (Mohale et al., 2014). 63

Bambara groundnut shell constitutes the weight of the pod wall after the grain is removed and is 64

highly variable depending on the pod yield and shelling percentage and sometimes could reach 65

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up to 1000 kg ha-1

. Musa et al. (2016) reported 374-896 kg ha-1

of shell weight which 66

corresponds to about 28.6-34.2 % of the total pod weight. Kishinevsky et al. (1996) reported a 67

shelling percentage of 59-76 %, which translates to about 648-1107 kg ha-1

( 24-41 % of the pod 68

weight) considering the average pod yield of 2700 kg ha-1

reported by Collinson et al. (1999). 69

Mwale et al. (2007) reported pod yield of 2980 kg ha-1

and a shelling percentage of 80 %, which 70

corresponds to a shell weight of 596 kg ha-1

(20 % of the total pod weight). Despite the high 71

weight of the crop’s shells, information on the utilization of this potential is limited and is 72

seldom reported as a yield component due to lack of a specific value (Mwale et al., 2007). There 73

is need to explore alternative ways to promote the utilization of this product which could address 74

the problem of crop waste disposal and promote the integration of the crop into new production 75

areas. Crop residue disposal in the sub-humid tropical regions such as Malaysia and Indonesia 76

has been attracting a major global concern due to inappropriate methods adopted by the resource 77

poor farmers, particularly in Indonesia, where farmers resort to the use of open burning as a 78

cheap way of clearing crop residues from the farm and thereby causing air pollution (haze) to 79

the neighboring countries and the entire region (Varkkey, 2013; Forsyth, 2014). 80

Pyrolysis is currently one of the most promising thermochemical processes for converting 81

biomass materials to products with high energy potentials. Typical pyrolysis products are bio-oil, 82

bio-char and non-condensable gas (Alburquerque et al., 2016; Gómez et al., 2016). Distribution 83

of pyrolysis products depends heavily on the process parameters such as pyrolysis temperature, 84

heating rate and vapor residence time in the reactor. There are different types of pyrolysis 85

namely; slow, intermediate and fast pyrolysis. Slow pyrolysis is also referred to as 86

carbonization. It is carried out at a temperature up to 400 °C, solid residence time of 1 h to 87

several hours, with product distribution of about 35 % bio-char, 30% bio-oil, and 35% non-88

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condensable gas. Fast pyrolysis can produce up to 80 % bio-oil, 12 % bio-char, and 13 % non-89

condensable gas at temperature around 500 °C, high heating rate, a short vapor residence time of 90

about 1 s, and rapid cooling of volatile (Bridgwater, 2012; Mohammed et al., 2015a). For 91

intermediate pyrolysis, 500-650 oC pyrolysis temperature and vapor residence time of 92

approximately 10 to 30 s are typical operating conditions. About 40-60% of the total product 93

yield is usually bio-oil, 15-25% bio-char and 20-30% non-condensable gas. Intermediate 94

pyrolysis produce bio-oil with less reactive tar which can be used directly as fuel in engines and 95

boilers, and dry char suitable for both agricultural and energy applications (Tripathi et al., 2016). 96

To the best of our knowledge, thermochemical conversion of Bambara groundnut shells has not 97

been reported in the literature. In this study, intermediate pyrolysis of Bambara shell was 98

investigated in a fixed bed reactor and pyrolysis products were characterized. 99

2. Materials and methods 100

Shells from two Bambara groundnut landraces, Karo (RKO) and EX-Sokoto (SKT) were 101

collected from Crops for the Future (CFF) field research Centre Semenyih, Selangor, Malaysia. 102

The material was dried at 105 °C in electrical oven. The dried materials were then shredded in a 103

Retsch® rotor beater mill (SM100, Retsch GmbH, Germany) to particle sizes between 0.2 mm 104

and 2.5 mm and stored in airtight plastic bags for further analysis. Volatile matter and ash 105

content on dry basis were determined. Fixed carbon was computed from the remaining bone dry 106

sample mass. The biomass ash elemental composition was analyzed with energy dispersive X-107

ray EDX (X-Max 20 mm2,

Oxford Instruments, UK). Higher heating value (HHV) was 108

determined using an oxygen bomb calorimeter (Parr 6100, Parr Instruments, Molin, USA). 109

Elemental compositions were determined using CHNS/O analyzer (2400 Series II CHNS/O 110

analyzer, Perkin Elmer Sdn Bhd, Selangor, Malaysia). The nature of chemical functional groups 111

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were evaluated with Fourier transform infrared spectroscopy (Spectrum RXI, PerkinElmer, 112

Selangor, Malaysia) using the potassium bromide (KBr) method. Potassium bromide (KBr) disc 113

(13 mm diameter translucent) was made from a homogenized 2 mg sample in 100 mg KBr using 114

a bench press (Carver 43500, Carver Inc.,USA) by applying 5.5 tonnes load for 5 min. Spectra 115

were recorded with Spectrum V5.3.1 software within wave number range of 400 to 4000 cm-1

at 116

32 scans and a resolution of 4 cm-1

. Thermogravimetric study was carried out using 117

thermogravimetric simultaneous thermal analyzer (TGA) (STA 6000, Perkin Elmer Sdn Bhd, 118

Selangor, Malaysia) in nitrogen atmosphere, flow rate 20 mL/min at temperatures between 30-119

950 oC and heating rate of 20

oC/min. About 10 mg (particle size of 0.2 mm) of sample was used. 120

Intermediate pyrolysis study was conducted at 500, 550, 600 and 650 oC in a vertical fixed bed 121

pyrolysis system as shown in Figure 1. The system consists of a fixed bed reactor made of 122

stainless steel (115 cm long, 6 cm inner diameter), a distribution plate with 1.5 mm hole diameter 123

which sit at 25 cm from the bottom of the tube, two nitrogen preheating sections, a cyclone, oil 124

collector, gas scrubbers and water chiller operating at 3 oC attached to a coil condenser. 200 g of 125

shell (bone dried, 2.5mm particle size) was placed on a gas distribution plate inside the reactor 126

tube. The setup was heated in a vertical furnance at 50 oC/min under 5 L/min nitrogen flow. The 127

reaction temperature was monitored with a thermocouple (K-type, NTT Heating, Sdn Bhd, 128

Selangor, Malaysia). The reaction time was kept at 60 min after the temperature attained the set-129

value. Pyrolysis vapor was condensed by passing through a condenser attached to the water 130

chiller and condensate (bio-oil) was collected in a container. Bio-oil, bio-char and non-131

condensable gas yields were calculated using Eq (1), (2) and (3). The experiment was repeated in 132

triplicates and stanadard deviations were computed. 133

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(1) -------- 100feed biomass ofweight

collected oil-bio theofweight

oil-bio(wt%) Yield

134

(2) ------ 100 feed biomass ofweight

collectedchar -bio theofweight

char-bio(wt%) Yield

135

(3) ----(2) Eq Eq(1)100gas econdensabl-non

(wt%) Yield 136

137

Characterization of bio-oil, bio-char and non-condensable gas were carried out accordingly using 138

analytical instruments as summerized in Table 1. FTIR of bio-oil was determined using a pair of 139

circular demountable KBr cell windows (25 mm diameter and 4 mm thickness). Detail chemical 140

composition of the bio-oil was determined using a gas chromatograph-mass spectrometer (GC-141

MS) system equipped with a quadruple detector and column (30m x 0.25mm x 0.25µm). The 142

oven was programmed at an initial temperature of 40 °C, ramp at 5 °C /min to 280 °C and held 143

there for 20 min. The injection temperature, volume, and split ratio were 250 °C, 1 µL, and 50:1 144

respectively. Helium was used as carrier gas at a flow rate of 1 mL/min. The peaks of the 145

chromatogram were identified by comparing with standard spectra of compounds in the National 146

Institute of Standards and Technology library (NIST, Gaithersburg, USA). 147

The composition of non-condensable pyrolysis product was monitored offline. The gas sample 148

was collected in a gas sample bag (Tedlar, SKC Inc., USA) and its composition analyzed using a 149

gas chromatography equipped with stainless steel column (Porapak R 80/100) and thermal 150

conductivity detector (TCD). Helium was used as a carrier gas and the GC was programed at 60 151

oC, 80

oC and 200

oC for oven, injector and TCD temperature respectively. Bio-char ultimate 152

analysis and inorganic composition were carried out following the procedure outlined for the 153

feedstock characterization. Crystallographic structure in the produced bio-char was examined 154

between 2Ө angle of 10°–70° at 25 mA, 45 kV, step size of 0.025°and 1.0 s scan rate. Surface 155

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and structural characteristics of the bio-char were also determined . 156

3. Results and Discussion 157

3.1 Characterization of feedstock 158

Characteristic of the feedstock is summarized in Table 2. Moisture content of SKT and RKO (as 159

received) was 4.67 and 7.12 wt%. Higher volatile matter (76.24 wt%) and lower ash content 160

(8.98 wt%) was recorded for the RKO compared to 73.83 wt% volatile matter and 10.10 wt% 161

ash content in the SKT. Similarly, the ultimate analysis revealed that RKO has carbon and 162

oxygen content of 37.36 wt% and 49.11 wt%, while 34.63 wt% carbon and 51.93 wt% oxygen 163

was recorded for the SKT. RKO has higher heating value (HHV) of 20.46 MJ/kg relative to 164

19.19 MJ/kg recorded for SKT. The higher energy content recorded for the RKO could be 165

attributed to its lower ash and oxygen contents. These physiocchemical characteristics are similar 166

to properties of peanut shell reported by Zhang et al. (2011). The authors recorded high volatile 167

matter and heating value of 71.5 wt% and 18.1 MJ/kg respectively. Their ultimate analysis result 168

showed higher carbon (47 wt%), and lower oxygen (34 wt%) and hydrogen (6 wt%) contents 169

compared to that of the Bamabara shells. EDX analysis of the ash showed that the inorganic 170

element in the RKO and SKT is in the following order potassium (K)>silica (Si)>Aluminum 171

(Al)>phosphorus (P)>Chlorine (Cl)>iron (Fe)>calcium (Ca)>magnesium (Mg) and 172

K>Cl>Si>Al>P>Mg respectively. The proportion of metallic elements in the RKO is similar to 173

that of peanut reported by Kuprianov and Arromdee (2013). The authors also reported 174

distribution of minerals in tamarin shell which is comparable to that of SKT except for the Si, 175

which is higher in SKT. The result of thermogravimetric analysis (TGA) analysis is shown in 176

Figure 2. The thermogravimetric (TG) and derivative thermogravimetric (DTG) profiles of both 177

samples exhibited similar decomposition characteristics. The residual weight at the end of the 178

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decomposition process was found to be 23 and 29 wt% for RKO and SKT respectively. From the 179

DTG curves, two main peaks from ambient to 100 oC and from 240 to 420

oC, which is attributed 180

to evaporation of moisture and decomposition of holocellulose (hemicellulose and cellulose) 181

component of the biomass. Hemicellulose which normally decomposes around 250 oC in a 182

lignocellulosic material was not observed in this study. It means that Bambara shells have less or 183

infinitesimal amount of hemicellulose. A noticeable peak around 200 oC was also observed, 184

which is ascribed to the decomposition of extractives. The pattern observed at the temperature 185

above 420 oC is attributed to the decomposition of lignin.This region exhibited unsteady thermal 186

degradation, which is ascribed to the presence of different oxygen functional groups in the lignin 187

(Mohammed et al., 2015b). TGA analysis of almond shell by Grioui et al. (2014) showed a 188

residual weight fraction of 27.5 wt%. The DTG curve exhibited main peak at temperature 189

between 220 and 400 oC, which was attributed to the decomposition of cellulose and 190

hemicellulose. The decomposition temperature values observed in this study are aslo similar to 191

those of other lignocellulosic biomass in the literature (Mohammed et al., 2015c). 192

3.2 Pyrolysis product distribution and physicochemical properties of the bio-oil 193

Product distribution from the pyrolysis of RKO and SKT at temperature between 500 and 650 194

oC, 5 L/min nitrogen flow and heating rate 50

oC/min is shown in Fig. 3. From Figure 3(a), total 195

bio-oil yield from RKO increased from 33.26 to 37.21 wt% as the temperature increased from 196

500 to 600 oC. This observation could be attributed to the decomposition of more lignin at such a 197

high temperature. At 650 oC, the oil yield dropped to 27.69 wt%, which is due to secondary 198

reactions of pyrolysis vapor at the elevated temperature (Mohammed et al., 2015a). The bio-oil 199

collected consist of organic phase and an aqueous phase. The yield of organic phase increased 200

from 9.8 wt% to 15.0 wt% at the temperature between 500 and 600 oC and latter declined to 9.3 201

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wt% at 650 oC. On the other hand, a continuous decrease in the aqueous phase from 24.3 to 18.4 202

wt% was recorded. Generally, the aqueous phase bio-oil is a product of holocellulose 203

decomposition whose yield and composition is greatly affected by the inorganic minerals in the 204

source biomass. The significant reduction in the bio-oil aqueous phase and increased non-205

condensable yield observed could be attributed to the synergistic effect of the most abundant 206

minerals in the biomass ash, which are Al and K (Table 2) since Si is an inert material and may 207

not possess any catalytic activity (Mohammed et al., 2016a). The yield of non-condensable gas 208

was 32.43 and 32.57 wt% at 500 and 550 oC but increased rapidity to 33.28 and 43.11 wt% at 209

600 and 650 oC. The corresponding bio-char yield was 32.96, 32.26, 29.51 and 29.20 wt%. The 210

high yield of non-condensable gas observed above could be mainly due to the secondary 211

reactions of pyrolysis vapor at the elevated temperature (Mohammed et al., 2015a) since there 212

was no substantial reduction in the biochar yield. Figure 3b shows pyrolysis product distribution 213

from the SKT. The total bio-oil yield was 26.17, 32.18, 32.78 and 27.58 wt% at 500, 550, 600 214

and 650 oC respectively. Similar to RKO, two phase bio-oil was collected from the pyrolysis of 215

SKT and the corresponding organic and aqueous phases recorded were 5.90, 10.84, 11.17, 5.77 216

wt% and 20.27, 21.34, 21.61, 21.81 wt%. The bio-char yield decreased from 39.91 wt% at 500 217

oC to 28.56 wt% at 650

oC while the corresponding yield of non-condensable gas increased from 218

33.92 to 43.86 wt%. The high yield of non-condensable gas could be a resultant effect of a 219

secondary reaction of pyrolysis vapor and devolatilization of the bio-char at high temperature. 220

Comparing the product distribution between RKO and SKT, the highest total bio-oil yield of 221

37.21 wt% was recorded at 600 oC from RKO compared to 32.78 wt% from SKT. A substantial 222

decrease in the biochar yield (11.35 %) with increasing temperature (500-650 oC) was observed 223

from SKT compared to 4.06 % recorded from the RKO. Higher non-condensable gas yield was 224

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also recorded with SKT at all temperatures relative to that obtained from RKO. These 225

observations can be attributed to the differences in the characteristics of the 226

feedstocks(Mohammed et al., 2016b), as summerized in Table 2. Comparing the pyrolysis 227

product distribution with products yield from other shells, Zhang et al. (2011) reported a total 228

pyrolysis oil of 60 wt% from peanut shell at 500 oC with corresponding 19 wt% char and 15 229

wt% non-condensable gas. Grioui et al. (2014) reported pyrolysis products distribution of 40 230

wt% bio-oil , 47 wt% gas and 13 wt% solid. Pyrolysis oil yield from other biomass species have 231

been reported by Lim et al. (2015) and (2016). They recorded bio-oil yield of 41.91-57.21 wt% 232

and 37.98-43.66 wt% from Napier grass and sago waste respectively at 600 oC pyrolysis 233

temperature. The variation in the products distribution can be linked to physicochemical and 234

structural characteristics of the respective feedstock. Physicochemical properties of Bamabara 235

shell bio-oil are summarized in Table 3. The pH value of organic phase from RKO and SKT was 236

4.4 and 3.3, which indicate that the oils are less acidic relative to the almond bio-oil (pH = 3.1) 237

reported by Grioui et al. (2014) (Table 3). Similarly, density of the bio-oils are relatively lower 238

than the values reported in the literature for almond and peanut bio-oil. Ultimate analysis result 239

(dry basis) calculated from the wet basis results using Eq (4), (5), (6) (de Miguel Mercader et al., 240

2010; Mohammed et al., 2016c) and (7) (Mohammed et al., 2015d) indicated that C, H, O and 241

HHV of the organic phase bio-oil from both Bambara shells are comparable to the elemental 242

composition of peanut and almond shell bio-oil. 243

)4(

100

21

wetC

dryC

OH

244

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)5(

100

21

AMOAMH2

AMH2

2wetH

dryH

OH

OH

245

)6(

100

21

AMOAMH2

AMO

2wetO

dryO

OH

OH

246

)7(

100

21

100

21wetLHV

dryHHV

OH

LHOH

W

247

C, H and O is carbon, hydrogen and oxygen content in wt %; ηH2O is the amount of water in the 248

oil (wt %); AMH and AMO is atomic mass of hydrogen and oxygen; LHw is the latent heat of 249

vaporization of water (2.2MJ/kg) 250

251

3.3 Fourier-Transform Infra-Red (FTIR) 252

FTIR spectra of chemical species in the bio-oil are shown in Figure 4. The common broad peak 253

around 3439 cm-1

implies that the samples contain chemical compounds with hydroxyl group (254

HO ) such as water, alcohols and phenol (Bordoloi et al., 2015). The peaks between 2838 and 255

2948 cm-1

common to the organic phase bio-oils (Figure 4a and 4b) is HC stretching 256

vibration, which indicate the presence of saturated hydrocarbon in the organic phase while the 257

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peak between 2041 and 2132 cm-1

common to all the samples is ascribed to the NC 258

functional group (Guo et al., 2015). Vibrations observed between 1625 cm-1

and 1707 cm-1

in all 259

the oil phases are attributed to OC , which signifies the presence of aldehydes, ketones or 260

carboxylic acids. The vibration at a frequency of 1515 cm-1

in both organic phases is ascribed to 261

CC bond while the band around 1454 cm-1

in all the samples is ascribed to HC bending, 262

suggesting the presence of alkenes/aromatic hydrocarbons. The vibrations observed at 1364, 263

1230 and 1273 cm-1

signify NC stretching (Yorgun and Yıldız, 2015). The sharp band 264

around 1112, 1093, 1050 and 1025 cm-1

are due to OC vibration indicating the presence of 265

alcohol and esters. The fingerprint at 755 cm-1

is ascribed to aromatic HC bending vibrations 266

while the fingerprint between 655 and 531 cm-1

is ascribed to alkyl halide (Yorgun and Yıldız, 267

2015). 268

3.4 GC-MS analysis of the bio-oil samples 269

Identification of detail chemical compounds in the bio-oil samples collected at 600oC was carried 270

out by GC-MS. The chromatograms (Figure 5) show twenty (20) most abundant compounds in 271

the bio-oils. The analysis revealed that the organic phase from both RKO and SKT consist of 272

similar compounds such as alkenes, benzene derivatives, nitrogenated compounds, esters, 273

aldehydes, ketones and acids in different proportions. A trace of sugar (L-Galactose) was also 274

detected in the oil from RKO while none was observed in the oil from SKT. The aqueous phase 275

from RKO consists predominant sugars and fine chemicals while that from SKT consist mainly 276

acids, ketones, aldehyde and phenolics. In order to elucidate the variation of these compounds in 277

the oil samples, the compounds were further grouped to hydrocarbon(HC), esters (ES), phenolics 278

(PH), sugars (SGR), value added chemicals (VAC) and acid, aldehyde, ketones (AAK). From 279

Figure 6, based on the GC-MS peak area (%), 7.09 % HC and 52.17 % PH was recorded in the 280

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organic phase RKO while 66.06 %PH was observed in the SKT with 2.31 % HC. The high 281

content of SGR (30.69 %) and low content of AAK (7.04 %) in the aqueous phase from the RKO 282

compared to low SGR (4.42 % ) and high AAK (36.95 %) in the SKT is attributed to the 283

catalytic effect of the respective biomass ash. Studies have shown that alkaline metals tend to 284

catalyze pyrolysis reactions which usually lead to degradation and polymerization of pyrolysis 285

product intermediate (Deshmukh et al., 2015; Bordoloi et al., 2015). Potassium (K) has been 286

identified to have impacts on both the pyrolysis product distribution and the composition of bio-287

oil. Studies by Shimada et al. (2008); Nowakowski et al. (2008); Mohammed et al. (2016a) have 288

demonstrated that potassium strongly promoted the formation of low molecular weight 289

compounds and inhibited the formation of sugar during biomass pyrolysis. Similar observations 290

have also been reported by Wang et al. (2010) and Trendewicz et al. (2015) during the pyrolysis 291

of pine wood and cellulose respectively. They also reported greater phenol yield with K as a 292

catalyst during the pyrolysis. Therefore, the high proportion of PH, AAK and lower content of 293

SGR in the oil from the SKT relative to RKO is ascribed to the higher K content in the SKT 294

(72.57 wt% ) relative to 44.96 wt% in the RKO (Table 2). Compositions of bio-oil from almond 295

shell bio-oil were mainly HC (alkanes and oleifins) as reported by Grioui et al. (2014) while 296

peanut shell bio-oil constitutes about 59 % PH and 31 % AAK (Zhang et al., 2011). Lim et al. 297

(2016) reported that bio-oil from Napier grass and sago waste consititute mainly PH and AAK 298

and PH, AAK and trace of HC respectively. The authors linked the difference in the chemical 299

composition to the presence of different mineral elements in the respective biomass. 300

3.5 GC analysis of the non-condensable gas sample 301

Composition the non-condensable gas collected at 600 oC is summarized in Table 4. The gas 302

constitutes a high percentage of methane (CH4), hydrogen (H2), carbon monoxide (CO) and 303

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carbon dioxide (CO2). The high composition of CH4 signifies thermal cracking mechanisms that 304

produce small organic molecules during the pyrolysis. The proportion of CH4 recorded in the 305

gas from RKO (9.43 vol%) was higher than that obtained from the SKT (7.42 vol%). This 306

suggests that there is interaction between the minerals Al (19.61 wt%) and K (44.96 wt%) since 307

Si is an inert material, (Table 2) in the biomass ash promoted the conversion of CH4 precursors 308

to form hydrocarbons. This observation is in good agreement with the higher percentage of total 309

hydrocarbons recorded in the bio-oil from RKO (18.28 % in both phases) relative to SKT (2.31 310

%). On the other hand, higher composition of H2, CO and CO2 in the gas from SKT was recorded 311

compared to RKO and could be linked to the higher percentage of K in the SKT ash. Study by 312

Wang et al. (2010) has shown that K promotes production of H2, CO and CO2 during pyrolysis 313

of lignocellulosic biomass. 314

3.5 Characteristics of produced bio-char 315

The properties of bio-char collected at 600 oC (Table 5) shows that both RKO and SKT produced 316

bio-char rich in carbon. The atomic ratio, H/C and O/C were 0.03 and 0.14 in RKO-char while 317

0.01 and 0.13 were recorded in SKT-char. The relative lower ratios observed in the SKT-char 318

indicates that SKT produced higher carbonized char which connotes a greater degree of 319

aromaticity. Comparing this characteristic with the individual biomass feedstock, H/C, O/C of 320

RKO and SKT biomass was 0.3, 1.3 and 0.33, 1.5 (Table 2). The decrease in the ratio, 321

particularly, the O/C in the produced bio-char was due to dehydration reaction during the 322

pyrolysis (Al-Wabel et al., 2013). 323

FTIR of the produced bio-char was compared with the source biomass as shown in Figure 7. The 324

averaged spectra showed similar characteristic peaks for both RKO and SKT biomass. Similarly, 325

common characteristic peaks were observed for the produced biochars. The peaks detected 326

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between 3723 and 3505 cm-1

in the feedstocks are assigned to different hydroxyl groups ( HO ), 327

principally from the cellulose component of the biomass. The peak at 3334 cm-1

is ascribed to 328

HN functional group, indicating the presence of nitrogenous group in the feedstock while the 329

peak observed at 3165, 3111 and 2960 cm-1

are due to HC stretching vibrations of cellulose, 330

hemicellulose and lignin in the biomass samples (Usman et al., 2015). These peaks disappeared 331

in the produced bio-chars, confirming the decomposition of structural component of the biomass 332

feedstock (Mimmo et al., 2014) while that observed around 3700 cm-1

due to HO could be 333

attributed to moisture adsorbed during sample preparations (Chen et al., 2016). The vibration at 334

1658 and 1588 cm-1

in the feedstock and produced bio-char is due to CC stretching while the 335

band at around 1417 cm-1

in all the samples is ascribed to HC bending. The band observed in 336

the feedstock at 1246 and 1033 is ascribed to CC O and OC stretching vibrations of 337

polysaccharides while the fingerprint at 545 and 462cm-1

is due to alkyl halides (Mohammed et 338

al., 2015b). These peaks completely disappeared in the produced bio-char confirming the 339

degradation of polysaccharides and alkyl halides during pyrolysis. 340

Physisorption analysis (Table 5) revealed that the produced bio-char is a porous material with a 341

pore volume of 0.01 cm3/g (RKO-char), 0.2 cm

3/g (SKT-char) with corresponding specific 342

surface area (BET) of 1.2 and 1.72 m2/g. This observation is also evident in the SEM micrograph 343

(Figure 8) which showed cracked, uneven and visible porous structure in the bio-char due to the 344

high degree of devolatilization at high temperature (600 oC) during the pyrolysis. 345

XRD analysis of the feedstocks and the produced bio-char is presented in Figure 9. The 346

diffractogram showed main characteristic peak for both RKO and SKT biomass at 22.23o and 347

22.09o respectively, which represent the crystalline component in the biomass, mainly cellulose 348

(Timpano et al., 2015). This peak completely disappeared in the produced bio-char, 349

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confirming complete decomposition of cellulose during the pyrolysis. New sets of peaks were 350

identified in the produced bio-chars around 26, 28, 30, 31, 40 and 50o. XRD search and 351

match reveal that the bio-char constituted predominantly sylvite mineral represented by the 352

peaks in both chars around 28.63o, 40.65

o and 50.43

o (Usman et al., 2015). The peak observed 353

around 26.83o and 30

o, 31

o indicate the presence of quartz and calcite (Yuan et al., 2011). 354

Thermogravimetic analysis (Figure 10a) of the produced bio-char showed a total residual weight 355

of 85.38 wt% and 86.49 wt% at 600 oC for SKT-bio-char and RKO-bio-char respectively 356

compared to the individual feedstock with 37.67 wt% and 32.47 wt%. This result indicates that 357

almost all the volatile matters in the biomass were completely volatilized during the pyrolysis. 358

This observation is also confirmed in the DTG plot (Figure 10b) as all the peaks in the feedstock 359

completely disappeared at 600 oC and above. The weight loss recorded at 600

oC in the bio-char 360

(13.51 wt%-RKO-char, 14.62 wt%-SKT-char) is due to combustion of the remaining organic 361

material in the bio-char. At higher temperature (900 oC), the total weight loss was 78.77 wt% in 362

RKO-char and 76.76 wt% in SKT-char, which correspond to about 7.72 wt% and 8.62 wt% 363

weigth loss between 600 and 900 oC . This observation could be attributed to the decomposition 364

of inorganic materials in the biochar at higher temperature (Azargohar et al., 2014). 365

4. Conclusion 366

Pyrolysis of Bambara groundnut shell was carried out in a fixed bed reactor. This study evaluates 367

impact of temperature on the pyrolysis product distribution (bio-oil, bio-char and non-368

condensable gas). It assesses the chemical and physicochemical properties of bio-oil, 369

characteristics of bio-char and composition of the non-condensable gas. The bio-oil collected 370

was a two-phase liquid, organic and aqueous phase. The organic phase from both shells have 371

high energy content (25-29 MJ/kg) and could be used directly as a fuel in boilers or a precursor 372

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for quality biofuel production. The aqueous phase particularly from RKO was predominantly 373

made up of sugars and other value-added chemicals, which can be used as a raw material for 374

production of consumer products and fine chemicals. Bio-char was a porous material rich in 375

carbon, which may well be used as a solid fuel. The bio-char also consist of macronutrient and 376

may be applied as a source of nutrient for plant growth. Non-condensable gas consists of 377

methane, hydrogen, carbon monoxide and carbon dioxide which can be recycled to the reactor as 378

an energy supplement for the endothermic pyrolysis reaction or as a carrier gas. Using Bambara 379

shells as feedstock for bioenergy production will encourage more production of the groundnut, 380

which will bring about food security, particularly in challenging environments where major 381

crops are severely limited by adverse climatic conditions in addition to income generation. 382

Acknowledgments 383

The project was supported by the Crops for the Future (CFF) and University of Nottingham 384

under the grant BioP1-005. 385

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